Lansing Lamont, a journalist and author whose “Day of Trinity” was a tour de force account of the scientists who helped conceive, develop and carry out the atom bomb tests that helped end World War II and initiate the nuclear age, died Sept. 3 at his home in New York. He was 83.

The cause was cancer, said his wife, Ada Jung Lamont.

Mr. Lamont was the scion of a distinguished banking family. His grandfather, Thomas W. Lamont, was a presidential adviser, philanthropist and board chairman of J.P. Morgan & Co. His father, Thomas S. Lamont, was a vice president at Morgan.

A notable exception to the patrician circle was an uncle, Corliss Lamont, a humanist philosopher who grew active in Socialist and civil liberties causes, drawing the ire of the red-baiting Sen. Joseph R. McCarthy.

Lance Lamont, as he was known, covered domestic and foreign politics for Time magazine in the 1960s and early 1970s, including the assassinations of President John F. Kennedy and presidential candidate Robert F. Kennedy.

Mr. Lamont wrote well-received books about the state of higher education (“Campus Shock,” 1979) and the potential political and economic quake caused by Quebec separatists (“Breakup: The Coming End of Canada and the Stakes for America,” 1994).

His most enduring text was his first, “Day of Trinity,” published in 1965. Richard Rhodes, a Pulitzer Prize-winning historian of the atomic bomb, called Mr. Lamont’s book “a breakthrough” because most of the documents about the Manhattan Project, the code name for the Allied effort to build an atomic device, were not declassified until a decade later.

“The only other book of note on the subject was ‘Brighter Than a Thousand Suns’ ” — written in the mid-1950s by the German-born author Robert Jungk — “and it had been basically a book that implied the Americans were guilty of their own holocaust, too,” Rhodes said. “There hadn’t been anything objective on the subject to that time, and the primary reason was because it was still a secret.”

The atomic bomb test was carried out in a stretch of New Mexico’s desert that the conquering Spaniards piquantly called Jornada del Muerto, or Journey of Death. Mr. Lamont interviewed many of the aging scientists who had worked at the Los Alamos, N.M., testing facility, hurling the world toward an age of nuclear proliferation.

Rhodes said the book remains relevant because few, if any, of those high-echelon scientists are still alive. “He produced a remarkable book,” Rhodes said, “and he did it with eloquence and stylistic grace.”

“Day of Trinity” provided fresh glimpses of the leading scientists of the age going about their lives at the Los Alamos laboratory: Enrico Fermi bicycling to work; J. Robert Oppenheimer brooding over the name for the test. Oppenheimer chose “Trinity” after reading a sonnet by John Donne about death and resurrection.

The narrative gains rollercoaster velocity as the final preparations are made for testing on July 16, 1945.

Mr. Lamont’s passage about the bomb test begins:

“A pinprick of brilliant light punctured the darkness, spurted upward in a flaming jet, then spilled into a dazzling cloche of fire that bleached the desert to a ghastly white. It was precisely 5:29:45 a.m.”

“Across the test site everything suddenly became infinitely tiny. Men burrowed into the sand like ants. . . . For a fraction of a second the light in that bell-shaped fire mass was greater than any ever produced before on earth.”

The temperature at the blast’s center was four times that of the sun’s core, the radioactivity emitted was 1 million times bigger than the world’s radium supply.

People for hundreds of miles around the blast were understandably alarmed. The government, Mr. Lamont dryly noted, issued an official explanation that an ammunition dump exploded.

Oppenheimer likened himself to “a destroyer of worlds.” George Kistiakowsky, an authority on explosives who was an architect of the bomb’s trigger, expressed apprehension about what he helped unleash: “I am sure that at the end of the world, in the last millisecond of the earth’s existence, humanity will see what we have just seen.”

Within weeks of the test, the Americans dropped two of the bombs over Japan and brought World War II to an end.

Lansing Lamont was born March 13, 1930, in New York. An older brother, Thomas W. Lamont II, was assumed lost at sea when his submarine, the USS Snook, went missing in the Pacific during the final months of World War II.

Mr. Lamont graduated in 1948 from the private Milton Academy in Massachusetts and four years later from Harvard University, where he was involved in the Hasty Pudding theater troupe. He served three years in the Army, then received a master’s degree from Columbia University’s journalism school in 1958.

He joined Time in 1961 and worked from Washington as a national correspondent. He was deputy chief of Time’s London bureau from 1969 to 1971. He subsequently was chief Canadian correspondent and then United Nations bureau chief and writer for the magazine’s World Affairs section.

In the 1980s and early 1990s, Mr. Lamont was director of Canadian affairs for the Americas Society, a group largely funded by David Rockefeller that sponsor lectures, conferences and publications about Western Hemisphere affairs.

At his death, he was board chairman of the American Trust for the British Library. He wrote a memoir, “You Must Remember This,” published in 2008.

Survivors include his wife of 59 years, Ada Jung Lamont of New York; four children; a brother; a sister; and 12 grandchildren.

On the 40th anniversary of the atomic bomb detonation, the New York Times printed Mr. Lamont’s editorial about the legacy of Trinity.

“In retrospect, little turned out the way the excited young scientists at Trinity has hoped it would,” he wrote. “The bomb they exploded would be the ultimate weapon, they thought — one so frightening it was inconceivable it could ever be used in anger after Hiroshima and Nagasaki.

“It would cause a profound change in the psyche of warriors, liberating them from the very idea of future wars,” he added. “Few of the Trinity scientists envisioned today’s state of the killing art, the numbing diversity of nuclear weapons, the sad reality that the bomb has become yet another item in the warrior’s inventory.”

Adam Bernstein has spent his career putting the "post" in Washington Post, first as an obituary writer and then as editor. The American Society of Newspaper Editors recognized Bernstein’s ability to exhume “the small details and anecdotes that get at the essence of the person” and to write stories that are “complex yet stylish.”

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